In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by bringing an electrographic developer into contact with the latent image. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the types of materials included in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Such elements may contain material which facilitates the migration of generated charge toward the oppositely charged surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a charge-transport material.
Among the various known types of electrophotographic elements are those generally referred to as multiactive elements (also sometimes called multilayer or multi-active-layer elements). Multiactive elements are so named, because they contain at least two active layers, at least one of which is capable of generating charge in response to exposure to actinic radiation and is referred to as a charge-generation layer (hereinafter sometimes alternatively referred to as a CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is referred to as a charge-transport layer (hereinafter sometimes alternatively referred to as a CTL). Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CGL or CTL. The CGL comprises at least a charge-generation material; the CTL comprises at least a charge-transport material; and either or both layers may additionally comprise a film-forming polymeric binder.
Among the known multiactive electrophotographic elements, are those that are particularly designed to be reusable and to be sensitive to imagewise exposing radiation falling within the visible and/or infrared regions of the electromagnetic spectrum. Reusable elements are those that can be practically utilized through a plurality (preferably a large number) of cycles of uniform charging, imagewise exposing, optional development and/or transfer of electrostatic latent image or toner image, and erasure of remaining charge, without unacceptable changes in their performance. Visible and/or infrared radiation-sensitive elements are those that contain a charge-generation material that generates charge in response to exposure to visible and/or infrared radiation. Many such elements are well known in the art.
For example, some reusable multiactive electrophotographic elements that are designed to be sensitive to visible radiation are described in U.S. Pat. Nos. 4,578,334 and 4,719,163, and some reusable multiactive electrophotographic elements that are designed to be sensitive to infrared radiation are described in U.S. Pat. Nos. 4,666,802 and 4,701,396.
A problem can occur when the CTL has been adventitiously exposed to blue and/or ultraviolet radiation (i.e., radiation of a wavelength less than about 500 nanometers, which, for example, forms a significant portion of the radiation emitted by typical fluorescent room lighting). This can occur, for example, when the electrophotographic element is incorporated in a copier apparatus and is exposed to typical room illumination during maintenance or repair of the copier's internal components. The problem is manifested as a buildup of residual potential within the electrophotographic element over time as the element is exercised through its normal cycles of electrophotographic operation after having been adventitiously exposed to blue and/or ultraviolet radiation.
For example, in normal cycles of operation such an element might be initially uniformly charged to a potential of about −500 volts, and it might be intended that the element should then discharge, in areas of maximum exposure to normal imagewise actinic visible or infrared exposing radiation, to a potential of about −100 volts, in order to form the intended electrostatic latent image. However, if the electrophotographic element has been adventitiously exposed to blue and/or ultraviolet radiation, there will be a buildup of residual potential that will not be erased by normal methods of erasing residual charge during normal electrophotographic operation. For example, after about 500 cycles of operation, the unerasable residual potential may be as much as −200 to −300 volts, and the element will no longer be capable of being discharged to the desired −100 volts. This results in a latent image being formed during normal operation that constitutes an inaccurate record of the image intended to be represented. In effect, the element has become no longer reusable, after only 500 cycles of operation.
It is known that all charge transporting materials absorb blue and/or ultraviolet light. Some charge transporting materials such as tri-p-tolylamine (TTA), absorb light and undergo a photochemical reaction. TTA in a CTL with bisphenol-A polycarbonate binder strongly absorbs ultraviolet light and the subsequent TTA photochemical reaction causes a buildup of residual potential with electrophotographic cycling as described above.
It is an object of the present invention to provide an electrophotographic photoconductor improved in stability to exposure to blue and/or ultraviolet light by using an additive hitherto unknown for addition to electrophotographic photoconductors.